# Successive approximation help!

Discussion in 'Homework Help' started by simon.cathy, Dec 18, 2006.

1. ### simon.cathy Thread Starter New Member

Dec 18, 2006
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I am constructing binary words from a given voltage, using successive approximation. What happens when the voltage is the SAME as a bit value(ie not greater, or less than).Hope that makes sense! Thanks.

2. ### Ron H AAC Fanatic!

Apr 14, 2005
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I would think that the comparators in most successive approximation A/Ds have hysteresis (Google), which precludes an indeterminate logic output level at the expense of a small amount of error. If no hysteresis is present, the succeeding latch (or flip-flop) can possibly go into a metastable state (Google).

3. ### beenthere Retired Moderator

Apr 20, 2004
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Hi,

If you get the spec sheet for one, the manufacturer usually does a good job of explaining the theory behind the A to D's operation. Analog Devices makes a zillion of them.

4. ### simon.cathy Thread Starter New Member

Dec 18, 2006
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A quick look at Analog Devices tutorials shows a `greater than or equal to` sign, which solves my problem! Thanks for the pointer.
Simon

5. ### Dave Retired Moderator

Nov 17, 2003
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Having the two inputs of a comparator the same is pretty difficult to achieve in practice still one must account for the possibility. Ron H has hit the nail on the head with the assessment of the comparator being in terms of a hysteresis characteristic. The comparator has two thresholds VTL and VTH which define the low and high input voltages respectively required to trigger a switch at the output. In fact in the context of ADCs a comparator without this hysteresis characteristic is pretty useless. Consider the situation with high-frequnecy noise (say from interference) imposed on the low-frequency signal you are comparing to some datum signal. If there were no hysteresis characteristic (i.e. VTL = VTH), then at the the theshold point the output of the comparator would fluctuate wildly in line with the high-frequency noise as it fluctuated around the threshold point, thus making the output reading null. In this sense the comparator is performing a high-frequnecy filtering function.

Dave